This invention relates to a slurry composition and a method of its preparation. In particular, the slurry composition of the present invention includes a silica wherein the silica comprises a surface modification. The silica-based slurry of the present invention is suitable for polishing articles and especially useful for chemical-mechanical planarization (xe2x80x9cCMPxe2x80x9d) of semiconductor and other microelectronic substrates.
In general, a plurality of integrated circuits is formed on a semiconductor substrate in fabricating a semiconductor wafer. The integrated circuits are typically formed by a series of process steps in which patterned layers of materials, such as conductive, insulating and semiconducting materials, are formed on the substrate. The use of copper and tantalum metal interconnects on semiconductor substrates is known in the art. In general, copper serves as an electrically conductive interconnection that is surrounded by an insulating interlayer dielectric material (ILD) such silicon dioxide, and tantalum serves as a barrier between copper and the ILD to prevent copper migration into the ILD. CMP is a technique known for removing such metallic materials from a semiconductor wafer. The control of metal removal rates, and selectivity between copper, tantalum, and ILD, for example, is desirable for achieving planarity requirements.
The planarization of a rough surface of an article such as a semiconductor substrate, to a smooth surface generally involves rubbing the rough surface with the work surface of a pad using a controlled and repetitive motion. Thus, the process typically involves rotating the pad and semiconductor wafer substrate against each other in the presence of a fluid. The fluid may contain a particulate material such as alumina, ceria, or silica or mixtures thereof. The pad and particulate material tend to mechanically planarize the semiconductor substrate, while the fluid and particulate material tend to chemically planarize the substrate and to facilitate the removal and transport of abraded material off and away from the rough surface of the article. The particulate material has both a chemical and mechanical role in planarizing the article.
In order to maximize the density of integrated circuits per wafer, it is necessary to have an extremely planar substrate at various stages throughout the semiconductor wafer production process. As such, semiconductor wafer production processes typically involve at least one, and typically a plurality of CMP steps. One such semiconductor process is the damascene, or the related dual damascene, process for integrated circuits that use copper interconnects. Another example is the shallow trench isolation process. Another example is the production of tungsten via on a silicon wafer.
It is known in the art to use alumina and silica in the CMP process. U.S. Pat. No. 5,980,775 discloses a CMP composition that includes an oxidizing agent, at least one catalyst, at least one stabilizer and a metal oxide abrasive such as alumina or silica. Further, this patent discloses a method for using the CMP composition to polish at least one metal layer of a substrate. U.S. Pat. No. 6,136,711 discloses a CMP composition which includes a compound capable of etching tungsten, at least one inhibitor of tungsten etching, and a metal oxide abrasive such as alumina or silica. Further, this patent discloses a method for using the CMP composition to polish tungsten-containing substrates. U.S. Pat. No. 5,904,159 discloses a polishing slurry comprising a silica-dispersed solution obtained by dispersing fumed silica particles in an aqueous solvent, wherein the average primary particle size is from 5 to 30 nm, having a light scattering index of from 3 to 6 and a silica concentration of 1.5% by weight, and an average secondary particle size of from 30 to 100 nm on a weight basis.
Further, surface modification of silica is known in the art. According to the method of Kovxc3xa1ts as described in Synthesis 1990, 1027, hydrated silica powder may be surface modified by heating with dialkylaminosilanes. U.S. Pat. Nos. 3,334,062; 4,143,027; 4,849,022; 5,008,305; and 5,902,635 also disclose methods of surface-modifying silica by mixing dried or partially hydrated silica with silanes. U.S. Pat. Nos. 1,110,331; 5,789,514; 5,9908,660 and 5,919,298 disclose methods of producing surface modified silica by aqueous phase condensation of reactive organic silanes. Moreover, the preparation of surface modified colloidal silica is disclosed in U.S. Pat. No. 2,786,042. U.S. Pat. Nos. 3,720,532; 4,068,024 and 4,443,357 disclose methods of surface modification of inorganic oxides by condensation with alcohol. Further, the modification of silica powder by gas phase condensation with organic silanes is disclosed in U.S. Pat. Nos. 4,015,031; 4,554,147; and 5,902,636. The modification of silica in a solvent phase by condensation with organic silanes is disclosed in U.S. Pat. Nos. 3,634,288; 3,768,537 and 5,647,962.
In general, the use of alumina abrasives has been considered desirable in the art because alumina particles have lower chemical reactivity than silica particles on silicon dioxide, and thus, alumina particles demonstrate a higher metal selectivity than silica particles. Without high selectivity, undesirable amounts of the silicon dioxide layer may be polished away with the metal. However, alumina slurries are generally more costly, and more prone to defects than silica slurries. Generally, alumina particles are more difficult to disperse than silica particles. Thus, it is desirable to develop a silica containing slurry that exhibits controlled removal rates and high selectivity relative to various metallic materials. xe2x80x9cSelectivityxe2x80x9d herein refers to the ratio of removal rates of two or more materials during CMP. For example, the selectivity of copper to tantalum represents the ratio of the removal rate of copper to the removal rate of tantalum.
Based on the prior art, one having ordinary skill in the art would expect metal removal rates for CMP to be described in accordance with the Preston Equation:
RR=KP+C
wherein RR represents removal rate, K is a constant referred to as the Preston Constant, P represents pressure at constant velocity, and C represents removal rate at zero pressure. As used herein, the term xe2x80x9cPrestonianxe2x80x9d refers to a relationship between removal rate and pressure that is described by the Preston Equation, i.e. removal rate increases monotonically with pressure at a constant velocity.
Modifications to the Preston Equation have been discussed in prior art references for the purpose of obtaining a better xe2x80x9cfitxe2x80x9d of data (cf Luo, Q.; Ramarajan, S. and Babu, S. V., Thin Solid Films 1998, 335, 160 and Ramarajan, S. and Babu, S. V., Proc. MRS Spring Meeting, San Francisco, Calif., April, 1999). However, these modified Preston Equations also describe Prestonian behavior.
The slurries of the prior art, which are described by the Preston Equation, result in undesirable amounts of dishing and erosion during the CMP process. Thus, it is also desirable to develop a slurry that minimizes dishing and eroding.
It has now been found that slurry compositions having the defined characteristics of the present invention provide performance advantages relative to metal removal rates and selectivity. Further, it has been found that a slurry composition of the present invention demonstrates performance that is not described by the Preston Equation, i.e., non-Prestonian behavior.
In accordance with the present invention, there is provided a slurry composition for CMP which comprises a surface-modified silica. The present invention also includes a method of preparing the slurry.
The features that characterize the present invention are pointed out with particularity in the claims that are part of this disclosure. These and other features of the invention, its operating advantages and the specific objects obtained by its use will be more fully understood from the following detailed description and the operating examples.
Other than in the operating examples, or where otherwise indicated, all numbers or expressions, such as those expressing structural dimensions, pressures, flow rates, etc, used in the specification and claims are to be understood as modified in all instances by the term xe2x80x9caboutxe2x80x9d.